1. Field of the Invention
This invention relates to polyurethanes, and, more particularly relates to non-blocking thermoplastic polyurethanes, their preparation and their use in medical devices.
2. Background of the Invention
Thermoplastic polyurethanes to be used as elastomers and the like have been known for a long time. Products prepared from polyisocyanates, high molecular weight polyetherglycols, and low molecular weight diols and diamines as chain extenders are conventionally referred to as polyetherurethanes, and this term, abbreviated PEU, will be used in this disclosure for polyurethanes having a polyether backbone.
PEU compositions develop microdomains conventionally termed hard segment domains and soft segment domains. They are (AB)n type block copolymers, A being the hard segment and B the soft segment, and are occasionally termed as segmented polyurethanes. The hard segment domains form by localization of the portions of the copolymer molecules which include the isocyanate and extender components whereas the soft segment domains form from the polyether glycol portions of the copolymer chains. The hard segments are generally more crystalline and hydrophilic than the soft segments and these characteristics prevail, in general, for the respective domains. One disadvantage of polyurethane resins of the softness desired for many medical devices, e.g., resins having Shore A hardness less than abut 100, is surface blocking (tack) after extrusion or molding into desired shapes. To avoid this problem, many remedies have been developed in the art including the use of external mold release agents and the use of various antiblockers or detackifiers in admixture with the polymer. Most antiblocking agents/detackifiers are low molecular weight materials which have a tendency to migrate or leach out of the polymer. This represents a problem when the polyurethanes are to be used as biomaterials (tubing, prostheses, implants, etc.). The presence of such low molecular weight extractables can affect the biocompatibility of the polyurethanes and lead to surface degradation, such as fissuring or stress cracking.
Although not related to biomaterials, U.S. Pat. No. 4,057,595 to Rauner et al. discloses a method for modifying the physical characteristics of polyurethane elastomers to reduce blocking wherein the polyurethane contains, within the polymer chain, a siloxane-polyoxyalkylene block copolymer.
Soft, non-blocking thermoplastic polyoxyalkylene polyurethanes having up to 15% of a soft segment formed from a polysiloxane devoid of oxygen atoms bonded to both silicon and carbon are disclosed by Zdrahala et al. in U.S. Pat. No. 4,647,643.
Fluorine containing polyurethanes are known. Kato et al., in Progress in Artificial Organs, 1983, page 858, discloses polyurethanes synthesized from fluorinated isocyanates. Yoon et al., in Macromolecules 19, 1068 (1986) discloses polyurethanes synthesized from fluorinated chain extenders. Field et al., in U.S. Pat. No. 4,157,358 discloses a randomly fluorinated epoxyurethane resin.
Although some progress has been made toward providing a thermoplastic polyurethane which is non-blocking without additives, which provides a desirable balance of stiffness in air and softness in liquid and which is suitable for blood contact, further improvement is needed. This invention is directed toward fulfillment of this need.